The L1 visa program forbids placing the employee at a third party site as part of a labor arrangement. Thus the L1 visa is not applicable to this sort of "training." I'm sure plenty of L1 employers play fast and loose with that rule because there is essentially no formal enforcement mechanism. Nevertheless, H1B is the official visa for that sort of work.
http://www.technologyexecutivesclub.com/Articles/outsourcing...